Mindef said that the new initiative was "implemented in response to feedback from some parents that enlistment information would be useful to them when their children were very young".
It is an "additional touch point" to parents, on top of the letters which the ministry sends out when their sons turn 13 and 16 ½.
The first, at age 13, is to inform the family that an exit permit is required if their son travels overseas for three months or more, and the second, to register for NS.
"As part of Mindef's ongoing engagement efforts to enhance awareness and understanding of NS commitments, we reach out to parents and pre-enlistees at various touch points to provide information on NS policies," a spokesman said.
Not all parents think that the letter at birth is necessary. Operations manager J.Y. Lim, 30, whose son was born last month, said: "It's kind of too early to inform us about NS, since the baby is just born, but I believe Mindef has its reasons."
Teacher Lim Lee Choon, 32, who also had a baby boy last month, felt the letter was informative. While she knew that every male Singaporean would need to serve NS, she "never knew that implications come on from as early as when they are 13", until she read the letter.
IT manager Andy Lee, 43, who has three sons aged 10, 12 and 14, said: "Some parents may find it a turn-off to be informed of their son's NS liability at such a young age.
"Still, I think there's no harm done. Some parents may have been thinking of relocating overseas with their children at an early age to help them avoid NS."
The issue of NS evasion was cast in the spotlight last month, when the High Court imposed harsher sentences for three NS defaulters, allowing the prosecution's appeal for a tougher punishment.
Among them was Ang Lee Thye, 43, who evaded NS for 231/2 years, and was jailed for two years and nine months - up from the initial sentence of two years.
Lawyer Laurence Goh, who has acted for NS defaulters, said the letter can help clear misconceptions.
"There are many who moved overseas at a young age and lived there for a long time. Maybe they got married with children and hold another country's citizenship," he said.
"But as long as they have not fulfilled their NS obligations, they can't renounce the Singapore citizenship." With the letter, parents cannot claim ignorance as an excuse, he added.
"They have to be fair to their children to ensure that they do not get into unnecessary trouble with the law," said Mr Goh.
MP Cedric Foo, a member of the Government Parliamentary Committee for Defence, said: "I see the letter at birth as a good reminder to parents about their sons' NS duties and key milestone dates...
"More information cannot be bad. Is it premature? I think not. It actually obviates the need for parents to do research on NS."